Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? There is a growing consensus among hurricane scientists that this is indeed quite possible. Two recent studies, by Zhao et al. (2009), "Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM", and by Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", found that global warming might increase wind shear over the Atlantic by the end of the century, resulting in a decrease in the number of Atlantic hurricanes. For example, the second study took 18 relatively coarse (>60 km grid size) models used to formulate the 2007 IPCC climate report, and "downscaled" them using a higher-resolution (18 km grid size) model called ZETAC that was able to successfully simulate the frequencies of hurricanes over the past 50 years. When the 18 km ZETAC model was driven using the climate conditions we expect in 2100, as output by the 18 IPCC models, the authors found that a reduction of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century resulted. An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

Figure 1. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

Since the Knutson et al. study using the 18 km resolution ZETAC model was not detailed enough to look at what might happen to major Category 3 and stronger hurricanes, a new study using a higher resolution model was needed. This was done by a team of modelers led by Dr. Morris Bender of NOAA's GFDL laboratory, who published their results in Science in February. The authors used the GFDL hurricane model--the model that has been our best-performing operation hurricane track forecasting model over the past five years--to perform their study. The GFDL hurricane model runs at a resolution of 9 km, which is detailed enough to make accurate simulations of major hurricanes. The researchers did a double downscaling study, where they first took the forecast atmospheric and oceanic conditions at generated by the coarse (>60 km grid) IPCC models, used these data to initialize the finer resolution 18 km ZETAC model, then used the output from the ZETAC model to initialize the high-resolution GFDL hurricane model. The final results of this "double downscaling" study suggest that although the total number of hurricanes is expected to decrease by the end of the century, we should expect an increase of 81% in the number of Category 4 and 5 storms in the Atlantic. This trend should not be clearly detectable until about 60 years from now, given a scenario in which CO2 doubles by 2100. The authors say that their model predicts that there should already have been a 20% increase in the number of Category 4 and 5 storms since the 1940s, given the approximate 0.5°C warming of the tropical Atlantic during that period. This trend is too small to be detectable, given the high natural variability and the difficulty we've had accurately measuring the exact strength of intense hurricanes before the 1980s.The region of the Atlantic expected to see the greatest increase in Category 4 and 5 storms by the year 2100 is over the Bahama Islands (Figure 2), since wind shear is not expected to increase in this region, and sea surface temperatures and atmospheric instability are expected to increase there.

The net effect of a decrease in total number of hurricanes but an increase in the strongest hurricanes should cause an increase in U.S. hurricane damages of about 30% by the end of the century, the authors compute, assuming that hurricane damages behave as they did during the past century. Over the past century, Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, Pielke et al., 2008.)

Figure 2. Expected change in Atlantic Category 4 and 5 hurricanes per decade expected by the year 2100, according to the Science paper by Bender et al. (2010).

CommentaryThese results seem reasonable, since the models in question have been successfully been able to simulate the behavior of hurricanes over the past 50 years. However, the uncertainties are high and lot more research needs to be done before we can be confident of the results. Not all of the IPCC models predict an increase in wind shear over the tropical Atlantic by 2100, so the increase in Category 4 and 5 hurricanes could be much greater. Also, the GFDL model was observed to under-predict the strength of intense hurricanes in the current climate, so it may not be creating enough Category 4 and 5 hurricanes in the future climate of 2100. On the other hand, IPCC models such as the UKMO-HadCM3 predict a very large increase in wind shear, leading to a drastic reduction in all hurricanes in the Atlantic by 2100, including Category 4 and 5 storms. So Category 4 and 5 hurricane frequency could easily be much greater or much less than the 81% increase by 2100 found by Bender et al.

The estimates of a 30% increase in hurricane damages by 2100 may be considerably too low, since this estimate assumes that sea level rise will continue at the same pace as was observed in the 20th century. Sea level rise has accelerated since the 1990s, and it is likely that this century we will see much more than than the 7 inches of global sea level rise that was observed last century. Higher sea level rise rates will sharply increase the damages due to storm surge, which account for a large amount of the damage from intense Category 4 and 5 hurricanes.

Keep in mind that while a 30% in hurricane damage by the end of the century is significant, this will not be the main reason hurricane damages will increase this century. Hurricane damages are currently doubling every ten years, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. By 2015, the authors expect the same hurricane would do $300 billion in damage. This number would increase to $600 billion by 2025 (though I think it is likely that the recent recession may delay this damage total a few years into the future.) It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing. Adoption and enforcement of strict building standards is also a must.

The authors of the GFDL hurricane model study have put together a nice web page with links to the paper and some detailed non-technical explanations of the paper.

Quoting skepticall2:Some people just don't understand cycles at all. If the Earth was supposed to be just one big average temperature sphere then they would find something else to complain about. Seriously the Earth has to go through cycles it is inevitable, to think it doesn't is just ignorant.

Doesn't this look like cycles? Also doesn't it look like the Earth has been much warmer than today? I know my SUV wasn't invented back then.

You make a very good point and observation. Everything in nature on this planet goes through cycles, especially climate. We could very well be part of yet another rotation in the cycle.

Whoever wrote that is a great doomcaster. He should come to JM's blog.

Lemme try... The first hurricane of 2010 is forecasted to form in the carribean in two weeks. It will intensify to a cat 5, then hit New Orleans, turn back into the GOM then hit Miami, then reintensify to a cat 9 and hit NYC.

The scenario, and apparently it was postulated by Lovelock, is certainly possible. However I disagree with the idea that China will neccesarily need to enter Siberia, as there are areas in Gansu and Shaanxi that should remain desert-free and are still sparsely populated. The population crash scenario is also a bit unrealistic, and my estimate would be closer to about 3.5 billion people by 2100. Some effects are inevetable but can be further abated using carbon capture technology, most of which is still "in the works". Even with that "saling above the Niagara falls" analogy, a crash could possibly be prevented by grabbing some oars and moving as quickly as possible towards the shore--but don't try this at home! By the way, your predictions, though probably sarcastic, are about 100x more unrealistic than mine. EVEN more than the predictions on the Hurricanes Wikia!

This is for Hydrus, for the threat of Severe Weather for tomorrow in TN :0)!!

Here is a portion of the Area Forecast Discussion out of Nashville, TN from earlier today:.PREV DISCUSSION... /ISSUED 347 PM CDT TUE APR 6 2010/

SHORT TERM...ANOTHER VERY WARM AFTERNOON IN PROGRESS ACROSS THEMID STATE, WITH 3 PM TEMPERATURES, ONCE AGAIN, BACK INTO THE LOWERAND MIDDLE 80S. WINDS WERE STRONG AND GUSTY OUT OF THE SOUTHWEST...SOMETIMES GUSTING TO AROUND 30 MPH. DUE TO THE LOW HUMIDITY ANDSTRONG WINDS, ENHANCED FIRE POTENTIAL EXISTS FOR THE REST OF THEAFTERNOON, AND SMOKERS SHOULD BE CERTAIN TO EXTINGUISH CIGARETTESIN A PROPER MANNER AND NOT TOSS SMOLDERING BUTTS ONTO THE GROUNDOR OUT OF CAR WINDOWS.

STILL LOOKS LIKE A COLD FRONT WILL SWEEP INTO THE AREA WEDNESDAYNIGHT AND EARLY THURSDAY, AND A FEW SHOWERS OR THUNDERSTORMS COULDEVEN WORK THEIR WAY INTO OUR MOST NORTHWESTERN COUNTIES BY LATETOMORROW AFTERNOON.

WE SHOULD GET LOCALLY HEAVY RAINFALL FROM THE BAND OF SHOWERS ANDSTORMS THAT MOVE THROUGH, WITH MOST AREAS RECEIVING BETWEEN THREEQUARTERS AND ONE INCH, AND LOCALLY HIGHER AMOUNTS WHERE THE STRONGERSTORMS OCCUR.

THERE IS A POTENTIAL FOR ISOLATED SEVERE THUNDERSTORMS WITH THISCOLD FRONT. ACCORDING TO SPC, THE PRIMARY CONVECTIVE MODE SHOULDBE LINEAR WITH POTENTIAL FOR BOW/LEWP FORMATIONS...AND PERHAPS AFEW EMBEDDED SUPERCELLS.

POST-FRONTAL SHOWERS AND CLOUDS WILL KEEP TEMPERATURES RELATIVELYCOOL ON THURSDAY, WITH HIGHS ONLY IN THE UPPER 50S TO MID 60S.

LONG TERM...WE COULD SEE SOME PATCHY FROST SATURDAY MORNING, AS COLDHIGH PRESSURE SETTLES IN ACROSS THE AREA, AND THOSE WITH OUTDOORTENDER VEGETATION SHOULD PLAN ON TAKING APPROPRIATE PRECAUTIONSSHOULD THE POSSIBILITY OF FROST BE ENTERED INTO THE FORECAST. BYEARLY NEXT WEEK, A WARMING TREND COMMENCES, WITH HIGHS PUSHINGBACK INTO THE 70S AND MAYBE EVEN PUSHING BACK TO AROUND 80 AGAIN INTHE LATTER PART OF THE PERIOD. EXTENDED PERIOD LOOKS DRY FOR SATURDAYTHROUGH WEDNESDAY.

In reading back through the comments here, through all the media articles, and through scientific journals, its becoming more and more evident to me that nobody out there has anywhere near a true understanding about the Global Warming issue since its such a complex issue in nature. So much evidence exists to support so many positions regarding global warming and so many different either personal or political agendas that there will never be a consensus. So, at least in my opinion, nobody should attack anyone for their views on the subject since there's no definitive scientific law, since in my eyes, I view global warming as a scientific theory.

Levi, I still disagree with you interpretation of the IPCC graphics which in my take is only slightly more moist conditions but taking into consideration the northwest I simply cannot find anything dramatic enough. And the Sahel runs from east Africa to West Africa. We can agree to disagree if you wish.